DESCRIPTION

SPF requires the SMTP client IP to match the envelope sender (return-path). When a message is forwarded through an intermediate server, that intermediate server may need to rewrite the return-path to remain SPF compliant. If the message bounces, that intermediate server needs to validate the bounce and forward the bounce to the original sender.

SRS provides a convention for return-path rewriting which allows multiple forwarding servers to compact the return-path. SRS also provides an authentication mechanism to ensure that purported bounces are not arbitrarily forwarded.

SRS is documented at http://spf.pobox.com/srs.html and many points about the scheme are discussed at http://www.anarres.org/projects/srs/

For a better understanding of this code and how it functions, please read this document and run the interactive walkthrough in eg/simple.pl in this distribution. To run this from the build directory, type "make teach".

METHODS

$srs = new Mail::SRS(...)

Construct a new Mail::SRS object and return it. Available parameters are:

Secret => $string

A key for the cryptographic algorithms. This may be an array or a single string. A string is promoted into an array of one element.

MaxAge

The maximum number of days for which a timestamp is considered valid. After this time, the timestamp is invalid.

HashLength => $integer

The number of bytes of base64 encoded data to use for the cryptographic hash. More is better, but makes for longer addresses which might exceed the 64 character length suggested by RFC2821. This defaults to 4, which gives 4 x 6 = 24 bits of cryptographic information, which means that a spammer will have to make 2^24 attempts to guarantee forging an SRS address.

HashMin => $integer

The shortest hash which we will allow to pass authentication. Since we allow any valid prefix of the full SHA1 HMAC to pass authentication, a spammer might just suggest a hash of length 0. We require at least HashMin characters, which must all be correct. Naturally, this must be no greater than HashLength and will default to HashLength unless otherwise specified.

Separator => $character

Specify the initial separator to use immediately after the SRS tag. SRS uses the = separator throughout EXCEPT for the initial separator, which may be any of + - or =.

Some MTAs already have a feature by which text after a + or - is ignored for the purpose of identifying a local recipient. If the initial separator is set to + or -, then an administrator may process all SRS mails by creating users SRS0 and SRS1, and using Mail::SRS in the default delivery rule for these users.

Some notes on the use and preservation of these separators are found in the perldoc for Mail::SRS::Guarded.

AlwaysRewrite => $boolean

SRS rewriting is not performed by default if the alias host matches the sender host, since it would be unnecessary to do so, and it interacts badly with ezmlm if we do. Set this to true if you want always to rewrite when requested to do so.

IgnoreTimestamp => $boolean

Consider all timestamps to be valid. Defaults to false. It is STRONGLY recommended that this remain false. This parameter is provided so that timestamps may be ignored temporarily after a change in the timestamp format or encoding, until all timestamps in the old encoding would have become invalid. Note that timestamps still form a part of the cryptographic data when this is enabled.

AllowUnsafeSrs

This is a backwards compatibility option for an older version of the protocol where SRS1 was not hash-protected. The 'reverse' method will detect such addresses, and handle them properly. Deployments upgrading from version <=0.27 to any version >=0.28 should enable this for MaxAge+1 days.

When this option is enabled, all new addresses will be generated with cryptographic protection.

Some subclasses require other parameters. See their documentation for details.

$srsaddress = $srs->forward($sender, $alias)

Map a sender address into a new sender and a cryptographic cookie. Returns an SRS address to use as the new sender.

There are alternative subclasses, some of which will return SRS compliant addresses, some will simply return non-SRS but valid RFC821 addresses. See the interactive walkthrough for more information on this ("make teach").

$sender = $srs->reverse($srsaddress)

Reverse the mapping to get back the original address. Validates all cryptographic and timestamp information. Returns the original sender address. This method will die if the address cannot be reversed.

$srs->compile($sendhost, $senduser)

This method, designed to be overridden by subclasses, takes as parameters the original host and user and must compile a new username for the SRS transformed address. It is expected that this new username will be joined on $SRSSEP, and will contain a hash generated from $self->hash_create(...), and possibly a timestamp generated by $self->timestamp_create().

$srs->parse($srsuser)

This method, designed to be overridden by subclasses, takes an SRS-transformed username as an argument, and must reverse the transformation produced by compile(). It is required to verify any hash and timestamp in the parsed data, using $self->hash_verify($hash, ...) and $self->timestamp_check($timestamp).

$srs->timestamp_create([$time])

Return a two character timestamp representing 'today', or $time if given. $time is a Unix timestamp (seconds since the aeon).

This Perl function has been designed to be agnostic as to base, and in practice, base32 is used since it can be reversed even if a remote MTA smashes case (in violation of RFC2821 section 2.4). The agnosticism means that the Perl uses division instead of rightshift, but in Perl that doesn't matter. C implementors should implement this operation as a right shift by 5.

$srs->timestamp_check($timestamp)

Return 1 if a timestamp is valid, undef otherwise. There are 4096 possible timestamps, used in a cycle. At any time, $srs->{MaxAge} timestamps in this cycle are valid, the last one being today. A timestamp from the future is not valid, neither is a timestamp from too far into the past. Of course if you go far enough into the future, the cycle wraps around, and there are valid timestamps again, but the likelihood of a random timestamp being valid is 4096/$srs->{MaxAge}, which is usually quite small: 1 in 132 by default.

$srs->time_check($time)

Similar to $srs->timestamp_check($timestamp), but takes a Unix time, and checks that an alias created at that Unix time is still valid. This is designed for use by subclasses with storage backends.

$srs->hash_create(@data)

Returns a cryptographic hash of all data in @data. Any piece of data encoded into an address which must remain inviolate should be hashed, so that when the address is reversed, we can check that this data has not been tampered with. You must provide at least one piece of data to this method (otherwise this system is both cryptographically weak and there may be collision problems with sender addresses).

$srs->hash_verify($hash, @data)

Verify that @data has not been tampered with, given the cryptographic hash previously output by $srs->hash_create(); Returns 1 or undef. All known secrets are tried in order to see if the hash was created with an old secret.

$srs->set_secret($new, @old)

Add a new secret to the rewriter. When an address is returned, all secrets are tried to see if the hash can be validated. Don't use "foo", "secret", "password", "10downing", "god" or "wednesday" as your secret.

$srs->get_secret()

Return the list of secrets. These are secret. Don't publish them.

$srs->separator()

Return the initial separator, which follows the SRS tag. This is only used as the initial separator, for the convenience of administrators who wish to make srs0 and srs1 users on their mail servers and require to use + or - as the user delimiter. All other separators in the SRS address must be =.

EXPORTS

Given :all, this module exports the following variables.

$SRSSEP

The SRS separator. The choice of = as internal separator was fairly arbitrary. It cannot be any of the following:

/ +

Used in Base64.

-

Used in domains.

! %

Used in bang paths and source routing.

:

Cannot be used in a Windows NT or Apple filename.

; | *

Shell or regular expression metacharacters are probably to be avoided.

$SRS0TAG

The SRS0 tag.

$SRS1TAG

The SRS1 tag.

$SRSTAG

Deprecated, equal to $SRS0TAG.

$SRSWRAP

Deprecated, equal to $SRS1TAG.

$SRSHASHLENGTH

The default hash length for the SRS HMAC.

$SRSMAXAGE

The default expiry time for timestamps.

EXAMPLES OF USAGE

For people wanting boilerplate and those less familiar with using Perl modules in larger applications.

Reverse Rewriting

NOTES ON SRS

Case Sensitivity

RFC2821 states in section 2.4: "The local-part of a mailbox MUST BE treated as case sensitive. Therefore, SMTP implementations MUST take care to preserve the case of mailbox local-parts. [...] In particular, for some hosts the user "smith" is different from the user "Smith". However, exploiting the case sensitivity of mailbox local-parts impedes interoperability and is discouraged."

SRS does not rely on case sensitivity in the local part. It uses base64 for encoding the hash, but allows a case insensitive match, making this approximately equivalent to base36 at worst. It will issue a warning if it detects that a remote MTA has smashed case. The timestamp is encoded in base32.

The 64 Billion Character Question

RFC2821 section 4.5.3.1: Size limits and minimums:

There are several objects that have required minimum/maximum
sizes. Every implementation MUST be able to receive objects
of at least these sizes. Objects larger than these sizes
SHOULD be avoided when possible. However, some Internet
mail constructs such as encoded X.400 addresses [16] will
often require larger objects: clients MAY attempt to transmit
these, but MUST be prepared for a server to reject them if
they cannot be handled by it. To the maximum extent possible,
implementation techniques which impose no limits on the length
of these objects should be used.
local-part
The maximum total length of a user name or other
local-part is 64 characters.

Clearly, by including 2 domain names and a local-part in the rewritten address, there is no way in which SRS can guarantee to stay under this limit. However, very few systems are known to actively enforce this limit, and those which become known to the developers will be listed here.

Cisco: PIX MailGuard (firewall gimmick)

WebShield [something] (firewall gimmick)

Invalid SRS Addresses

DO NOT MALFORMAT ADDRESSES. This is designed to be an interoperable format. Certain things are allowed, such as changing the semantics of the hash or the timestamp. However, both of these fields must be present and separated by the SRS separator character =. The purpose of this section is to illustrate that if a malicious party were to malformat an address, he would gain nothing by doing so, nor would the network suffer.

The SRS protocol is predicated on the fact that the first forwarder provides a cryptographic wrapper on the forward chain for sending mail to the original sender. So what happens if an SRS address is invalid, or faked by a spammer?

The minimum parsing of existing SRS addresses is done at each hop. If an SRS0 address is not valid or badly formatted, it will not affect the operation of the system: the mail will go out along the forwarder chain, and return to the invalid or badly formatted address.

If the spammer is not pretending to be the first hop, then he must somehow construct an SRS0 address to embed within his SRS1 address. The cryptographic checks on this SRS0 address will fail at the first forwarder and the mail will be dropped.

If the spammer is pretending to be the first hop, then SPF should require that any bounces coming back return to his mail server, thus he wins nothing.

Cryptographic Systems

The hash in the address is designed to prevent the forging of reverse addresses by a spammer, who might then use the SRS host as a forwarder. It may only be constructed or validated by a party who knows the secret key.

The cryptographic system in the default implementation is not mandated. Since nobody else ever needs to interpret the hash, it is reasonable to put any binary data into this field (subject to the possible constraint of case insensitive encoding).

The SRS maintainers have attempted to provide a good system. It satisfies a simple set of basic requirements: to provide unforgeability of SRS addresses given that every MTA for a domain shares a secret key. We prefer SHA1 over MD5 for political, rather than practical reasons. (Anyone disputing this statement must include an example of a practical weakness in their mail. We would love to see it.)

If you find a weakness in our system, or you think you know of a better system, please tell us. If your requirements are different, you may override hash_create() and hash_verify() to implement a different system without adversely impacting the network, as long as your addresses still behave as SRS addresses.

Extending Mail::SRS

Write a subclass. You will probably want to override compile() and parse(). If you are more familiar with the internals of SRS, you might want to override hash_create(), hash_verify(), timestamp_create() or timestamp_check().

CHANGELOG

MINOR CHANGES since v0.29

timestamp_check now explicitly smashes case when verifying. This means that the base used must be base32, NOT base64.

hash_create and hash_verify now explicitly smash case when creating and verifying hashes. This does not have a significant cryptographic impact.

MAJOR CHANGES since v0.27

The SRS1 address format has changed to include cryptographic information. Existing deployments should consider setting AllowUnsafeSrs for MaxAge+1 days.

MINOR CHANGES since v0.26

parse() and compile() are explicitly specified to die() on error.

MINOR CHANGES since v0.23

Update BASE32 according to RFC3548.

MINOR CHANGES since v0.21

Dates are now encoded in base32.

Case insensitive MAC validation is now allowed, but will issue a warning.

MINOR CHANGES since v0.18

$SRSTAG and $SRSWRAP are deprecated.

Mail::SRS::Reversable is now Mail::SRS::Reversible

This should not be a problem since people should not be using it!

You must use $SRS0RE and $SRS1RE to detect SRS addresses.

MAJOR CHANGES since v0.15

The separator character is now =.

The cryptographic scheme is now HMAC with SHA1.

Only a prefix of the MAC is used.

This API is still a release candidate and should remain relatively stable.